Conventionally, an integrated semiconductor laser and electric field absorption type light modulator (hereinafter may be referred to as simply "modulator") on an InP substrate has been employed as a signal light source for high-speed modulation optical communication.
FIGS. 5(a) to 5(i) are diagrams illustrating process steps of a method for producing an integrated semiconductor laser and light modulator that has a long wavelength semiconductor laser and an electric field absorption type light modulator, described in Journal of Lightwave Technology, Vol. 8, No. 9, September 1990, pp. 1357 to 1362. FIGS. 5(a) to 5(d) are cross sections of the process steps, and FIGS. 5(e) to 5(i) are perspective views thereof.
A description will be given of the process steps.
First, as shown in FIG. 5(a), a .LAMBDA./4-shifted diffraction grating with 240 nm pitch 10a is formed in a region A where a semiconductor laser is to be formed on the (100) surface of an n-type InP substrate 10.
Secondly, as shown in FIG. 5(b), there are successively grown on the (100) surface of the n-type InP substrate 10 by liquid phase epitaxy (hereinafter referred to as LPE), an n-type InGaAsP light guiding layer 11 having the thickness of 0.1 .mu.m and a bandgap energy corresponding to a wavelength .LAMBDA. of 1.3 .mu.m, an undoped InGaAsP active layer 12 having the thickness of 0.1 .mu.m and a bandgap energy corresponding to a wavelength .LAMBDA. of 1.57 .mu.m, an undoped InGaAsP buffer layer 13 having the thickness of 0.1 .mu.m and a bandgap energy corresponding to a wavelength .LAMBDA. of 1.3 .mu.m, and a p-type InP layer 14 having the thickness of about 1 .mu.m in this order, and further photoresist 15 is deposited on the p-type InP layer 14. After a portion of the photoresist 15 deposited on a region B where the light modulator is to be produced is removed by conventional photolithography, dry etching using the photoresist 15 as a mask is performed to the n-type InGaAsP Night guiding layer 11, the undoped InGaAsP active layer 12, and the undoped InGaAsP buffer layer 13 and the p-type InP layer 14. As shown in FIG. 5(c), the region (B) on the n-type InP substrate 10 where the light modulator is to be produced is exposed.
Next, as illustrated in FIG. 5(d), an undoped InGaAsP light absorbing layer 16 having the thickness of 0.3 to 0.5 .mu.m and a band energy corresponding to wavelength of 1.44 .mu.m, an undoped InGaAsP buffer layer 17 having a thickness of 0.1 to 0.3 .mu.m and a bandgap energy corresponding to a wavelength .LAMBDA. of 1.25 .mu.m, and a p-type InP cladding layer 18 having a thickness of about 3 .mu.m are successively produced by vapor phase epitaxy (hereinafter referred to VPE), and further photoresist 19 is deposited on the p-type InP cladding layer 18.
After patterning the photoresist 19 in a stripe shape in the light waveguide direction of the semiconductor laser by conventional photolithography technique, as shown in FIG. 5(e), dry etching is performed to the above-described semiconductor layers grown on the substrate 10 using the patterned photoresist 19 as a mask, and the layers are formed in a mesa stripe 20 having a width of 2 .mu.m.
Then, the undoped InGaAsP light absorbing layer 16, the undoped InGaAsP buffer layer 17 and the p-type InP cladding layer 18 which are grown on the region A on the substrate 10 where the semiconductor laser is to be formed are etched away, and a portion 18a of the InP cladding layer 18 grown on the region B on the substrate 10 where the light modulator is to be formed is etched away for electrical isolation, resulting in a state shown in FIG. 5(e).
Next, an Fe-doped InP layer 21 of high resistance is grown by VPE so as to bury the both sides of the mesa stride part 20 and the part 18a which is a portion of the InP cladding layer 18 etched away for electrical isolation as shown in FIG. 5(f), and an undoped InGaAs contact layer 22 is grown thereon by VPE.
P-type diffused regions 26a and 26b are respectively formed by selective diffusion of Zn at opposite sides of a stripe portion of the Fe-doped InP layer 21 and the undoped InGaAs contact layer 22 formed on the mesa stripe part 20, so that deepest parts of the p-type diffused regions reach the mesa stripe part 20. Then, a selective etching is carried out so that only a portion of the InGaAs contact layer 22 formed on the mesa stripe part 20 remains, thereby resulting in a state illustrated in FIG. 5(g).
Next, a SiN film 23 is deposited over the InGaAs contact layer 22 and the Fe-doped InP layer 21, and conventional photolithography and etching are performed to form openings 23a and 23b in the SiN film 23 as illustrated in FIG. 5(h).
Then, as shown in FIG. 5(i) a p-side electrode formation metal layer is forming on the SiN film 23 in the openings 23a and 23b, and this metal layer is patterned so that the portions of the metal layer buried in the openings 23a and 23b and peripheral portions thereof remain, thereby forming a p-side electrode 24a for the semiconductor laser and a p-side electrode 24b l for the light modulator.
Additionally, an n-side electrode 25 is formed on the rear surface of the substrate 10, completing an integrated semiconductor laser and light modulator on the same substrate.
In the above-described semiconductor laser with light modulator, the energy band gap of the undoped InGaAsP light absorbing layer 16 of the light modulator side is larger than that of the active layer 12 of the semiconductor laser side. Therefore the light generated in the active layer 12 in the mesa stripe part 20 of the semiconductor laser side propagates to the undoped InGaAsP light absorbing layer 16 of the light modulator side, and the laser beam is emitted from the cleaved facet of this undoped InGaAsP light absorbing layer 16. During this operation, by switching between applying a voltage and applying no voltage across the p-side electrode 24b of the light modulator side and the n-side electrode 25, i.e., switching between applying an electric field and applying no electric field to the undoped InGaAsP light absorbing layer 16, the energy band gap of the undoped InGaAsP light absorbing layer 16 becomes larger or smaller than that of the active layer 12 in the semiconductor laser. Thereby, the light generated in the active layer 12 is insuccessively cut off in the light modulator and emitted from the cleaved facet of the undoped InGaAsP light absorbing layer 16, generating an optical signal having a transmission capacity of, for example, 5 Gb/s.
The semiconductor laser with light modulator fabricated by the above-described process steps illustrated in FIGS. 5(a) to 5(i) involves the following problems. That is, because the light absorbing layer 16 of the light modulator and the active layer 12 of the semiconductor laser are produced on the semiconductor substrate 10 with different semiconductor layers grown by the different epitaxial growth processes, coupling loss unfavorably occurs when the light generated by the semiconductor laser propagates to the light modulator, thereby hardly outputting a stable optical signal.
On the other hand, it is are known that when semiconductor layers are epitaxially grown by metal organic chemical vapor deposition (hereinafter referred to as "MOCVD") in a state where a prescribed region on the semiconductor substrate is covered with an insulating film such as SiO.sub.2 film and SiN film, source gases directly supplied to the front surface of the semiconductor substrate are thermally decomposed on the substrate and epitaxially grow materials, while source gases supplied to the SiO.sub.2 film or SiN film do not react thereon, and are diffused over the SiO.sub.2 film or SiN film and move to a location where the semiconductor substrate is exposed, and are thermally decomposed on the semiconductor substrate to produce epitaxially grown material. During this epitaxial growth, due to the above-described nature, there is a difference in the amount of the existing source gases contributing to the epitaxial growth per unit time at a position close to the SiO.sub.2 film or SiN film and at a position remote from the film on the substrate. As a result, there is a difference in the growth speed of the semiconductor layers at a position close to the SiO.sub.2 film or SiN film and at a position remote therefrom, leading a difference in the thickness of the grown semiconductor layers, thereby resulting in thick semiconductor layers grown at a position close to the SiO.sub.2 or SiN film and thin semiconductor layers grown at a position remote from the film.
Recently, utilizing the nature as described above, that there occurs a difference in the thickness of the semiconductor layers when the semiconductor layers are epitaxially grown by MOCVD with a prescribed region of the substrate covered with the SiO.sub.2 or SiN film, a method of producing a semiconductor laser with light modulator that produces semiconductor layers of the semiconductor laser and those of the light modulator in the same epitaxial growth step has been provided.
Next, a description will be given of this production method.
FIGS. 6(a) to 6(c) are diagrams illustrating process steps of a method for producing a semiconductor laser with light modulator, described in ELECTRONICS LETTERS 7 Nov. 1991 Vol. 27, No. 23, pp. 2138 to 2140. FIGS. 6(a) and 6(b) are perspective views and FIG. 6(c) is a cross section. Enlarged views D and C in FIG. 6(b) respectively show layer structures of the semiconductor laser and the light modulator.
First, as shown in FIG. 6(a), a diffraction grating 31 is produced in a prescribed region on an InP substrate 30 where the semiconductor laser is to be produced (this side of paper surface of the figure is a region where the light modulator is to be formed). Then, SiO.sub.2 films 32 of a stripe shape parallel to the light waveguide direction of the semiconductor laser to be produced are formed so as to sandwich this diffraction grating 31. Secondly, as shown in FIG. 6(b), the semiconductor layers are successively grown by MOCVD in the order of InGaAsP light guiding layer 33a (semiconductor laser side) and 33b (light modulator side), InGaAs/InGaAsP multi-quantum well active layer 34a (semiconductor laser side) and 34b (light modulator side), and InP cladding layer 35a (semiconductor laser side) and 35b (light modulator side). Thereafter, as shown in FIG. 6(c), an InGaAsP layer is disposed on the InP cladding layers 35a and 35b, a prescribed portion of the InGaAsP layer is removed to form a separating groove 37, and InGaAsP cap layers 36a of the semiconductor laser side and 36b of the light modulator side are produced. Next, a p-side and an n-side electrode 38a and 38b are formed respectively on the InGaAsP cap layers 36a of the semiconductor laser side and 36b of the light modulator side. Further, an electrode of opposite conductivity type to that of the electrodes 38a and 38b is formed at the rear surface side of the substrate 30, completing the integrated semiconductor laser and light modulator on the same substrate.
In the semiconductor laser with light modulator obtained by such a production process, the thickness of the well layer 34c1 of the multi-quantum well active layer 34a of the semiconductor laser side is larger than that of the well layer 34c2 of the multi-quantum well active layer 34b of the light modulator side caused by the above-described nature, as illustrated in FIG. 6(b). Thereby, the energy band gap of the multi-quantum well active layer 34b of a light absorbing layer of the light modulator side is larger than that of the multi-quantum well active layer 34a of the semiconductor laser side. Therefore, the light generated in the multi-quantum well active layer 34a of the semiconductor laser side propagates to the multi-quantum well active layer 34b of the light modulator side and is modulated by the light modulator according to the same principle as the above-described semiconductor laser with light modulator shown in FIGS. 5(a) to 5(i).
In the semiconductor laser with light modulator fabricated by the prior art production process shown in FIGS. 6(a) to 6(c), since the active layer of the semiconductor laser side and the active layer of the light modulator side (i.e., the light absorbing layer) comprise a continuous semiconductor layer produced in a single process step, the laser beam generated in the semiconductor laser can propagate to the light modulator more efficiently relative to the semiconductor laser with light modulator fabricated by the production process shown in FIGS. 5(a) to 5(i). In such a method which, utilizing the difference in the amount of the existing source gases contributing to the epitaxial growth at position close to and at a position remote from the SiO.sub.2 film on the substrate, a large difference in the layer thickness is generated in the grown semiconductor layers thereby to form portions having a large energy band gap and a small energy band gap in the semiconductor layers. However, the thicknesses of the semiconductor layers of the semiconductor laser part and those of the semiconductor layers of the light modulator part are considerably different from each other. Therefore, the multi-quantum well active layers 34a and 34b are disposed on levels having a difference generated due to the difference in the thicknesses of the light guiding layers 33a and 33b as illustrated in FIG. 6(c). Therefore, there also occurs a difference in level between the multi-quantum well active layers 34a and 34b, and this difference in level interrupts the propagation of the laser beam, not sufficiently improving the propagation characteristic of the laser beam between the active layer of the semiconductor laser and the light absorbing layer of the light modulator.
When the semiconductor layers are epitaxially grown by MOCVD with the SiO.sub.2 film at a prescribed region on the semiconductor substrate, and the growth environment including, for example, the temperature and the pressure in a reactive tube changes during the growth by even only a little, the behavior of the source gases at the SiO.sub.2 film unfavorably changes to a great extent even if the flow rates of the source gases are constant. Therefore, it is impossible to produce by this method a difference in the thickness of the semiconductor layers grown, that produces a desired difference in the energy band gap with a good reproducibility, and therefore it is impossible to produce an integrated semiconductor laser and light modulator having a desired operation characteristic at a high yield.